The objective of this study was to document the reduced acute phase response that appears in children with viral as opposed to bacterial infections. The white blood cell count (WBCC), the erythrocyte sedimentation rate (ESR), and leukocyte and erythrocyte adhesiveness/aggregation were determined in 36 children with acute bacterial infection, 29 children with viral infection, and 19 controls. A significant reduced WBCC, ESR, and leukocyte and erythrocyte adhesiveness/aggregation was noted in the children with acute viral infection as opposed to those with bacterial infection: 10,800 +/- 3600 and 20,000 +/- 10,000 cells/cm2, 29 +/- 21 and 53 +/- 35 mm Hg, 23 +/- 9 and 41 +/- 15%, and 3.4 +/- 5.1 and 9.8 +/- 13.6 microns, respectively. The results indicate that a reduced acute phase response can be observed in children with an acute viral infection. This can have diagnostic implications and pathophysiological consequences in terms of less flow impairment in the microcirculation due to less red and white blood cell aggregation.